first make sure you're prewedging. It helps a lot. Then when you're dropping the box, don't cut all the way to the adjacent tooth. It's kinda hard to explain but think of leaving the shell of the tooth. That way, you can use your hand instruments. Use your margin trimmers and hatchets to make the margins and floor smooth.
At the beginning, you're going to use a lot of hand instruments but after you drop so many boxes you'll realize you use hand instruments for less than 10% of the prep. Hope it helps.
And yeah, you can use a diamond bur, but don't recommend that because in real life you're not going to be prepping a class II with a diamond bur. It will take way too long.
And make sure you're sitting on the correct position so you know when to stop using the handpiece.